1. Field of the Invention
The present invention relates to a radiotherapy system having a motion tracking device for recognizing and tracking the position of a target.
2. Description of the Related Art
For radiotherapy of irradiating a target in the body with treatment radiation, there has been described in, for example, Japanese Patent No. 3746747 (hereinafter, referred to as Patent Document 1), a radiotherapy system which uses a motion tracking device for recognizing a target position in a body and applies the treatment radiation on the basis of the recognized target position. The purpose of recognizing the target position is to irradiate a target as in the treatment plan even when there is a movement of the target due to respiration or the like.
As an irradiation method of the treatment radiation using the motion tracking device, there are intercepting irradiation in which irradiation is performed only where the target position coincides with a plan, and pursuing irradiation in which the direction of the treatment radiation is changed according to the movement of the target position. Patent Document 1 discloses an example of the pursuing irradiation. In any case, in order to correctly reproduce a dose distribution in a patient's body, there is a need to correctly position a patient (subject) inclusive of not only the target position but also peripheral organs. This is because when a substance in the passing area of radiation changes, a dose reaching the target changes. This is also done to avoid entering of a critical organ not to be irradiated with radiation into a radiation passing area.
In order to correctly position the patient, the positional relationship of the target in the patient's body, the radiation passing area and the critical organ or the like is required to be grasped. To this end, a three-dimensional X-ray CT image (computed tomographic image) is used. Patent Document 1 has described that a cone beam CT apparatus is used which performs imaging using an X-ray imaging device installed in a rotatable support device of the radiotherapy system.
Further, there is known Japanese Patent No. 4499087 (hereinafter, referred to as Patent Document 2) as a technology for photographing or capturing an image that enables the positional relationship of a target moved due to respiration, a radiation passing area and a critical organ to be grasped precisely in cone beam CT. Patent Document 2 describes: “Means for performing a cone beam CT scan to obtain a plurality of planar projection images of a patient, means for determining a respiration cycle of each projection image using the in-projection image feature of each of a plurality of projection images, and means for directly applying a respiration correlated technique to the projection image, on the basis of the determined respiration cycle.”